One path to short-term storage of solar energy is to use concentrated sunlight to split water, thereby converting solar energy (i.e., photons) into chemical energy in the form of hydrogen (H2) gas. This process is commonly referred to as photo-induced water-splitting. Additional benefits of H2 gas generation may include the use of hydrogen as a fuel for vehicles.
Numerous methods have been proposed to accomplish photo-induced water-splitting. One method focuses on running an electrolysis cell off of a photovoltaic power source. See, for example, R. McConnell et al., “Generating Hydrogen through Water Electrolysis Using Concentrator Photovoltaics,” National Renewable Energy Laboratory, Conference Paper NREL/CP-520-37093 (January 2005). Another approach involves electrolysis by immersion of a photovoltaic element in an aqueous solution. See, for example, T. Lopes et al., “An innovative photoelectrochemical lab device for solar water splitting,” Solar Energy Materials and Solar Cells, Vol. 128, pgs. 399-410 (June 2014).
These methods however are often inefficient as they compound the efficiency of photovoltaic conversion with the efficiency of the electrolysis process resulting in an overall efficiency that is poor relative to other means of hydrogen generation. Both methods also operate below 100 degrees Celsius (° C.), i.e., in liquid water, and thereby require that all of the energy for catalysis come from visible photons. Further, heat energy that evolves from the portion of incident light that is not photovoltaically converted to electricity is thermodynamically inaccessible.
Accordingly, more efficient techniques are needed for generating hydrogen from sunlight that utilize both incident visible light as well as heat, and thus ultimately result in higher operating efficiencies.